It is often times desirable to be able to provide a hard copy recording of video information outputted by an instrument that develops video signals. By way of example, protocol analyzers such as those made by Hewlett-Packard Company and identified as Model Nos. 4951 and 4952 make extensive use of video attributes such as inverse video, underline, special characters, and half bright. However, a conventional or standard printer, such as an Epson-compatible printer, is not able to accept and provide a hard copy recording of such video information. Rather, a special video printer is required, in order to print the representation of the video information. The special video printer can only be used to print or provide a hard copy recording of video-related information. Other information that is not video based, such as text information or hexadecimal based information, cannot be printed by the special video printer. Consequently, a separate and relatively expensive printer needs to be provided to print video information and another printer is required to print non-video related information. Applications software could be developed which provides printer support for printing some, but not all, video-based information obtained from the afore-mentioned protocol analyzers. However, it would be much more desirable to provide a solution that supports all of the operating modes of the aforesaid protocol analyzers and is able to provide an instantaneous hard copy or "snapshot" of the display.
In conjunction with devising a solution whereby a specialized video printer is not required to print video-related information, it would be advantageous to provide an accurate hard copy representation of the video information outputted by different instruments. Such instruments, however, differ in the number of dots or pixels that are displayed for each row that forms a part of the display screen. Consequently, it is difficult to devise a phase locked loop (PLL) circuit solution since such a circuit would require information relating to the number of dots provided on the display screen. Stated another way, a PLL-based solution is too dependent on the video information provided by a particular instrument and a more desirable solution is one that is independent of the video information provided by a particular instrument. Another possible solution for converting the video information to a format suitable for a conventional printer might involve reliance on the occurrence of dots on the display screen having a certain predetermined intensity whereby such dots having the predetermined intensity can be detected by a system and this information utilized to convert the video information to an acceptable format. This technique, however, would depend greatly on the occurrence of dots on the display screen having at least a predetermined intensity. If such dots did not occur with sufficient frequency, the accuracy of the representation of the video information would be diminished due to insufficient information or samples of the dots having the predetermined intensity.
It would be beneficial, therefore, to provide an apparatus that interfaces between an instrument outputting video information and a standard printer and which apparatus can be used with different instruments to provide a hard copy or snapshot of the display screen of whichever instrument is connected to the apparatus. Further, it would be worthwhile to provide such an apparatus at a relatively low cost while such apparatus could also convert all video information on the display screens to a format suitable for the standard printer and, particularly, for use with the protocol analyzer Model Nos. 4951 and 4952 of Hewlett-Packard.